Next question: If the C of G is normally supposed to be over centre line, why does the rider extend his knee to the inside of the corners? Is it to counteract the centrifugal force in order to keep the bike just a little more upright through the corners?

(Admittedly when you're on the bike you have to "do", not "think". But when off on the bike, it's worth trying to develop a better understanding of the forces and techniques involved).

Yup. The "knee-down" style was invented by a bloke called Paul Smart (BIL to Barry Sheene) while riding a crap bike with no ground clearance. It keeps the bike more upright for the same turning radius on a motor-bike. On a pushy, the inside knee must be bent to ensure the pedal is up. Pedal down on the inside will give you pedal strike and brown undies.

Jules, appreciate the diagram, but have you got centripetal confused with centrifugal?

The former is a force towards the centre of an imaginary circle formed by the turning arc. It is a pulling force that stops a rotating object from moving outwards.

The latter is a force away from the centre of the turning circle, and I think what you mean by 'not a real force'. It is more correctly referred to as a fictitious force because it is in actuality an inertial force, i.e. a moving object wants to travel in the same direction. When that object is constantly restrained from moving in a straight direction, its inertia is being countered.

"Fictitious' centrifugal force is in actuality lots of small moments of straight line inertial force by a turning object, that if not resisted would see it fly off at a tangent.

Centripetal force is the resistance to those lots of moments of straight line inertia.

I've always given up trying to better understand the physics of a cornering bike, but will revisit it. You have to get your head around centripetal acceleration and coefficients of friction before considering the centre of gravity and its relationship to a safe maximal lean angle.

PawPaw wrote: Jules, appreciate the diagram, but have you got centripetal confused with centrifugal?

yes, i think you're right. the diagram was always technically wrong, as you said, there is no such thing as centripetal or centrifugal force. but in representing a dynamic entity (cornering bike) as a static one, the centrifugal force effectively substitutes for what you feel while cornering as centripetal acceleration.

PawPaw wrote: I've always given up trying to better understand the physics of a cornering bike, but will revisit it. You have to get your head around centripetal acceleration and coefficients of friction before considering the centre of gravity and its relationship to a safe maximal lean angle.

it's pretty simple. the diagram i showed summarises it, except to be more accurate you would remove the "centripetal force" i added. that would throw the rider out of force equilibrium, resulting in the rider experiencing (centripetal) acceleration.

When I was a kid they taught us Latin roots. They don't do that any more. Now it's all about "functional grammar" and that's why we have such abominations as "quadbikes" and "homophobia". But I digress. Centrifugal is portmanteau Latin, a composite of "centrum" (centre) and the adjectival form of "fugere" (to flee), hence "flees from the centre". Genius!"Centripetal" is similarly constructed but uses "petere" (to seek).

jules21 wrote:it's pretty simple. the diagram i showed summarises it, except to be more accurate you would remove the "centripetal force" i added. that would throw the rider out of force equilibrium, resulting in the rider experiencing (centripetal) acceleration.

Simple as far as it goes. It begs the question "What about bicycle steering. How does that work?". It's interesting that bicycle steering developed entirely empirically which seems appropriate because that's the way you learn to take corners. In fact that's the way you learn to ride a bike. There isn't any theory involved. Counter steering is counter intuitive but it is fundamental to staying upright on a bike. It's no wonder those first few rides when we get it are so amazing.

notwal wrote:It begs the question "What about bicycle steering. How does that work?".

precession - it's not that complicated, but moreso than the simple lateral forces i drew.

Well as an explanation that's on a par with the Python's instructions on how to become a gynaecologist. In fact I don't know if gyroscopic effects (if that's what you mean by precession) have much to do with it apart from providing inertia.

As far as I can tell, if for eg you are riding in a straight line and want to turn left, you first counter-steer right. The bike starts falling to the left. The wheel flops to the left under the influence of castor and proceeds to try to catch up with the falling bike. The disequilibrium of this condition is balanced by the centripetal force as per your diagram. It's a bit like being in orbit around the Earth where you are continually falling but because you are going so fast you continually over shoot.

The calculation of the required geometry (castor and fork offset) from first principles is less than obvious to me. I think it's somewhat complicated, but IDK.

Ken Ho wrote: Yup. The "knee-down" style was invented by a bloke called Paul Smart (BIL to Barry Sheene) while riding a crap bike with no ground clearance. It keeps the bike more upright for the same turning radius on a motor-bike. On a pushy, the inside knee must be bent to ensure the pedal is up. Pedal down on the inside will give you pedal strike and brown undies.

OK. Next question: How does keeping the bike more upright confer an advantage? Does it simply increase the angle between the bike and the road, therefore allowing the bike to be lowered a little more during higher speeds or tighter cornering?

I don't think comparing to motorbikes is all that relevant...when you have a motorbike with little ground clearance you actually hang off the side of the bike or at least move around in your saddle a lot.Not really an option on a pushy...especially as you don't have drive out of the corner.There is also the keeping both feet level and squeezing the top tube with your legs technique...works just as well on certain types of corners....I am sure there are more.Having a good line is the most important...within reason if roads are not closed.But you want to start from the outer most section and cut the apex of the corner before drifting back out to the outer as you exit the corner...but yet again this is only done if you are riding at the limit.It is a soft science, different things work for different people...practice makes you better, but to be honest if you descending isn't a natural thing then it probably never will be.Some people are just technically better than others.Knowing the corner intimately (with out actually being on your arse!) will help get round it quicker if it is a descent you do often...but if you really want to push it be prepared for road rash.

Downhill wrote:OK. Next question: How does keeping the bike more upright confer an advantage? Does it simply increase the angle between the bike and the road, therefore allowing the bike to be lowered a little more during higher speeds or tighter cornering?

there is no simple explanation in physics for that. the "cornering force" is determined solely by the horizontal distance between the rider/bike's CoG and the tyre contact point on the road.

the answer really lies in how you use your body as a shock absorber. if you just plant yourself on the seat, and handlebars, you have little ability to absorb bumps or respond to any other inputs (slides, etc.) positioning your body is important - that's why MTBers stand on the pedals a lot. leaning over the bike kind of positions the rider to respond quickly to bumps, etc. that's my take anyway.

Consciously counter-steering is the key to fast, confident descending.

Not coming from a moto background I had no idea how to use it (although I'd heard of it) until taught by David Heatley one day coming down Mt Hotham.

Other crucial factors are1. Trim speed using only the rear brake through a corner (front brake straightens you up)2. Use the front+rear brakes together to wash off speed when in a straight line3. Push foot down on the outside4. Be on the drops - braking control is improved and the lower center of mass improves stability5. Lean the bike under you, not with you6. Follow other cyclists to learn ideal lines7. Take it easy on wet/gravel/debris-covered roads!8. Relax your upper body. The bike responds poorly to a stiff-armed rider.

and finally, a new tip I rarely see given:

9. Pull in your stomach! That is, tighten your transverse abdominal muscles, just lightly. This will help flatten your back and steadies you on the bike without stiffening your arms. It recruits the rest of the core, ensuring that the spine is protected. If you've ever experienced lumbar discomfort during a very long descent on the drops, then developing & using your TA will make a big difference.

I'd just take it easy, small steps and gradually increase the speed. And keep a light touch on the bars and weight over the front. That seems to keep the wobbles under control.

I keep low as I can as well - but to be perfectly honest, my limit is 60km/h - I don't like going faster than that. I'm not as heroic as some of these awesome riders who go flying past at 90km/h or better.

Last edited by g-boaf on Wed Apr 03, 2013 11:32 am, edited 1 time in total.

Another tip: the speed at which you descend isn't important, enjoying the descent is. I'm a relatively slow descender myself, and am quite happy with that, because if anything it makes the ride last longer, which is always a good thing .

g-boaf wrote:I'd just take it easy, small steps and gradually increase the speed. And keep a light touch on the bars and weight over the front. That seems to keep the wobbles under control, along with weight off the seat if I remember right.

Ah... no. Weight back on the saddle always gives you better balance. Putting your weight forward over the steering axis allows the rear end to steer the bike. I've seen the results of this on a descent with a guy who pushed his body forwards and had a heavy load on his rear panniers - it was not pretty. His heavy load only magnified the rear-steering and he was not able to control it.

Weight forward is fine if it is on the wheel (ie. front panniers). Not good if it's your body above the steering axis.

g-boaf wrote:I'd just take it easy, small steps and gradually increase the speed. And keep a light touch on the bars and weight over the front. That seems to keep the wobbles under control, along with weight off the seat if I remember right.

Ah... no. Weight back on the saddle always gives you better balance. Putting your weight forward over the steering axis allows the rear end to steer the bike. I've seen the results of this on a descent with a guy who pushed his body forwards and had a heavy load on his rear panniers - it was not pretty. His heavy load only magnified the rear-steering and he was not able to control it.

Weight forward is fine if it is on the wheel (ie. front panniers). Not good if it's your body above the steering axis.

VRE wrote:Another tip: the speed at which you descend isn't important, enjoying the descent is. I'm a relatively slow descender myself, and am quite happy with that, because if anything it makes the ride last longer, which is always a good thing .

I'm with you. I'm rubbish at descending (although a little bit better of recent times). The thought of popping a rear tyre at 75kmh? No thank you. I'm out there to enjoy my riding.

Having said that. practice helps A LOT. Or hope your partner says "would you like me to meet you at the summit and we can have coffee, then I'll drive you home?" Bonus

Last edited by takeitasread on Wed Apr 03, 2013 9:53 pm, edited 1 time in total.